Many laboratory studies on prenatal or developmental effects of methylmercury intoxication have utilized doses that have produced gross teratogenecity or no apparent effects. Two problems with this approach are: 1. Doses have been unrealistic compared to concentrations within the ecosystem. 2. Human fetal methylmercury intoxication (fetal Minamata disease) has been described as a non-specific cerebral palsy, characterized by mental retardation and motor disturbances. We propose to study the more subtle effects, if produced, by much lower doses of methylmercury administered acutely, at different stages of development, as well as chronically. With the use of procedures that have proven sensitive to drug and x-irradiation insult prenatally, in two classes of vertebrates, we hope to determine if critical periods exist for neurochemical, behavioral, and/or ultrastructural alterations as a consequence of exposure to methylmercury during development. In addition, if present, an attempt will be made to determine if prevention, reversal or attenuation of these effects can be accomplished with the use of heavy metal antidotes (i.e. dimercaprol or penicillamine). Finally, we propose to determine, by appropriate dose-response relationship studies, the dose or concentration of methylmercury that is compatible with normal development.